1. Field of the Invention
The present invention generally relates to a diversity scheme communication system and, more particularly, to a communication system using weak radio waves, i.e., a harmonic-frequency communication system with an improved diversity scheme for performing data communication using the fundamental and harmonic-frequency waves of electromagnetic waves generated from the output of a transmission side.
2. Description of the Related Art
In a radio communication system, the fundamental wave of electromagnetic waves is generally used. Such a communication system is suppressed by interference waves generated near the communication system. In order to solve this problem, there is provided a communication system with a frequency diversity scheme in which data communication is performed by superposing the same data on a plurality of carriers.
FIG. 8 is a block diagram showing a conventional diversity scheme communication system. Data D1 is input to first and second transmitting sections 1 and 2. The first transmitting section 1 modulates the data D1 and superposes it on a first carrier. The second transmitting section 2 modulates the data D1 and superposes it on a second carrier. As a result, the data D1 is radiated in the air as electromagnetic waves by a transmitting antenna 4 via an antenna combining equipment 3. The radiated electromagnetic waves travel in the air and are received by a receiving antenna 6. The received electromagnetic waves are input to a first receiving section 7 for the first carrier and to a second receiving section 8 for the second carrier, so that data are demodulated from the carriers and output. The quality levels of demodulated data are compared by some method, and one data of better quality is selected by a switch 9 and output therefrom as data D2.
In the conventional communication system, however, the following drawback is posed. That is, in addition to an antenna combining equipment, a plurality of transmitting sections are required for transmitting a plurality of carriers, thereby complicating the arrangement of the communication system.